This invention relates to an alloy having an excellent thermal fatigue resistance which is suitable for through-hole connections or flat mountings of electronic parts in or on printed substrates, hybrid substrates, etc., at temperatures below the melting point of Sn-40% by wt. Pb solder (solid phase, 183.degree. C.; liquid phase, 190.degree. C.).
The connection of elements or parts by passing lead wires through holes in a printed substrate and wave-soldering them with an Sn-40% by wt. Pb eutectic solder is a popular practice in the assembly of semiconductor parts and elements. However, in view of the recent trend toward high-density mounting and hybridization which requires the mounting of a large variety of multifunctional parts onto a printed board and, on the other hand, the necessity of removing and replacing parts in the event of a failure, it is fairly difficult to maintain the high reliability of the joints when using an Sn-40% by wt. Pb eutectic solder alone because of the necessity of re-melting joints (soldered zones) during the removal of demounting of parts. Accordingly, a new type of solder is required which can be used at a low melting point in combination with the conventional Sn-40% by wt. Pb solder, with a high reliability of the soldered joints, and also with special thermal fatigue resistance properties comparable with or even better than those of conventional solders.
Generally, soldering is performed at a temperature somewhat higher than the liquid-phase temperature of the solder to provide a good wettability with the soldered area. Any solder used in combination with conventional Sn-40% by wt. Pb solder must be treated at a working temperature which does not exceed the solid-phase temperature (183.degree. C.) of the Sn-40% by wt. Pb solder.
An attempt to lower the melting point by adding a third metal (Bi) to a Sn-Pb solder is disclosed in Japanese Patent Publication No. 36446/71. The solder of Example 1 (60% by wt. Sn-5% by wt. Bi-35% by wt. Pb) of this patent has a liquid phase temperature of 175.degree. C., and therefore it is impossible to use this solder to control the actual working temperature so that it does not exceed the solid phase temperature (183.degree. C.) of Sn-40% by wt. Pb solder. The solder compositions of Examples 2 and 3 have liquid phase temperatures of 200.degree. C. and 190.degree. C., respectively, which are well above 183.degree. C., so that their use in combination with the conventional Sn-40% by wt. Pb solder is also impossible.